System and method for monitoring and determining patient parameters from sensed venous waveform

ABSTRACT

Devices, systems, and methods for monitoring patient hemodynamic status, systemic vascular resistance, reversal of cardiac and respiratory rates, and patient respiratory volume or effort are disclosed. A peripheral venous pressure is measured and used to detect levels, changes, or problems relating to patient blood volume. The peripheral venous pressure measurement is transformed from the time domain to the frequency domain for analysis. A heart rate frequency is identified, and harmonics of the heart rate frequency are detected and evaluated to determine, among other things, hypovolemia or hypervolemia, systemic vascular resistance, and of cardiac and respiratory rates, and patient respiratory volume or effort.

PRIORITY CLAIM

This application claims priority to U.S. Provisional Application No. 62/671,108, entitled “System and Method for Monitoring and Determining Patient Parameters From Sensed Venous Waveform”, filed May 14, 2018, the entire contents of which are incorporated herein by reference and relied upon.

BACKGROUND

Conventional methods of establishing blood volume and related metrics regarding patient condition have relied upon highly invasive measurements, such as central venous pressure (herein “CVP”), pulmonary artery pressure, and/or capillary pressure. Such CVP measurements require the insertion of a catheter, specifically for the purpose of measuring blood pressure, within the central portion of the patient's circulatory system. In addition to being highly invasive, the insertion of a catheter solely for the purpose of pressure monitoring increases the complexity of treatment and raises the risk of complications, such as infection. Additionally, CVP measurements are believed to be slower to change in response to certain acute conditions, as the circulatory system attempts to compensate for blood volume disequilibrium (particularly with respect to hypovolemia) by protecting blood volume levels in the central circulatory system at the expense of the periphery. For example, constriction in peripheral blood vessels may reduce the effect of fluid loss on the central system, thereby masking blood loss for a period of time in conventional CVP measurements. Such masking can lead to delayed recognition and treatment of patient conditions, resulting in worse patient outcomes.

To address the issues associated with CVP measurements, the use of peripheral intravenous analysis (PIVA) has been developed, as described in U.S. paatent pplication Ser. No. 14/853,504 (filed Sep. 14, 2015 and published as U.S. Patent Publication No. 2016/0073959) and PCT Application No. PCT/US16/16420 (filed Feb. 3, 2016, and published as WO 2016/126856). Such PIVA techniques measure peripheral venous pressure (PVP) using intravenous (IV) lines, such as IV tubing which may or may not be attached to a saline drip, infusion pump, or dialysis pump. In addition to utilizing existing IV lines, the PIVA techniques also include transformation of the PVP measurements into the frequency domain to identify a respiratory rate frequency (F₀) equal to the respiratory rate of the patient and a heart rate frequency (F₁) equal to the heart rate of the patient. Although the PIVA techniques previously disclosed provide an excellent indication of heart rate and blood volume status in certain situations, the disclosure herein further improves upon the previously disclosed PIVA techniques to address challenges related to other situations, improve accuracy, provide earlier warnings of potential problems, and identify additional patient conditions.

SUMMARY

In light of the present disclosure, and without limiting the scope of the disclosure in any way, in a first aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, a blood volume evaluation system for monitoring a patient using a measurement associated with a peripheral venous pressure (PVP) within a peripheral vein of a circulatory system of the patient includes a PVP sensor and an evaluation unit. The PVP sensor includes a transducer disposed adjacent to or connected to an intravenous (IV) tube in fluid connection with the peripheral vein and configured to generate an electronic signal associated with the PVP. The evaluation unit includes a computer processor communicatively connected to the PVP sensor to receive the electronic signal and a memory storing non-transitory computer-readable instructions that, when executed by the computer processor, cause the evaluation unit to obtain a time-domain PVP signal, including a plurality of values of the electronic signal over a sample period. The evaluation unit then applies a transformation to the time-domain PVP signal to generate a frequency-domain PVP signal. The evaluation unit identifies a plurality of frequencies associated with local maxima of the frequency-domain PVP signal, including a heart rate frequency (F₁), a first harmonic frequency (F₂) at a first harmonic of the heart rate frequency, and a second harmonic frequency (F₃) at a second harmonic of the heart rate frequency. The evaluation unit calculates a blood volume metric based at least in part upon the heart rate frequency (F₁), the first harmonic frequency (F₂), and the second harmonic frequency (F₃).

In an aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, calculating the blood volume metric based at least in part upon the heart rate frequency (F₁), the first harmonic frequency (F₂), and the second harmonic frequency (F₃) includes the equation

${{PIVA}\mspace{14mu} {Score}} = {{c_{3}{\tanh \left( \frac{g_{0} + {g_{1}{mag}^{f\; 1}} + {g_{2}{mag}^{f\; 2}} + {g_{3}{mag}^{f\; 3}}}{2} \right)}} + {c_{2}{\tanh \left( \frac{h_{0} + {h_{1}{mag}^{f\; 1}} + {h_{2}{mag}^{f\; 2}} + {h_{3}{mag}^{f\; 3}}}{2} \right)}} + {c_{1}{\tanh \left( \frac{i_{0} + {i_{1}{mag}^{f\; 1}} + {i_{2}{mag}^{f\; 2}} + {i_{3}{mag}^{f\; 3}}}{2} \right)}} + c_{0}}$

where each of c₀, c₁, c₂, c₃, g₀, g₁, g₂, g₃, h₀, h₁, h₂, h₃, i₀, i₁, i₂, and i₃ are constants. Furthermore, mag^(f1) is the magnitude of the heart rate frequency (F₁), mag^(f2) is the magnitude of the first harmonic frequency (F₂), and mag^(f3) is the magnitude of the second harmonic frequency (F₃).

In an aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the blood volume metric is an indication of one of the following: hypovolemia, hypervolemia, or euvolemia.

In an aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the system further includes an alarm unit configured to present alarms to a user. The instructions further cause the evaluation unit to communicate an alarm indicator to a presentation unit when the blood volume metric indicates the blood volume of the patient is abnormal, the alarm indicator causing the presentation unit to present an alarm.

In an aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the alarm includes a recommendation of an action to be taken in response to the calculated blood volume metric.

In an aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the electronic signal is an electronic pressure signal indicative of pressure within the IV tube.

In an aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the transducer of the PVP sensor is a pressure sensor and is disposed in fluid connection with the interior of the IV tube. Furthermore, the interior of the IV tube is in fluid connection with the circulatory system of the patient via the peripheral vein.

In an aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, a device for measuring and evaluating blood volume of a patient, includes a peripheral venous pressure (PVP) sensor and an evaluation unit. The PVP sensor includes a transducer configured to generate an electronic signal associated with a pressure within a peripheral vein of a circulatory system of the patient. The evaluation unit includes a computer processor communicatively connected to the PVP sensor to receive the electronic signal and a memory storing non-transitory computer-readable instructions that, when executed by the computer processor, cause the evaluation unit to obtain a time-domain PVP signal, including a plurality of values of the electronic signal over a sample period. The evaluation unit then applies a transformation to the time-domain PVP signal to generate a frequency-domain PVP signal. The evaluation unit identifies a plurality of frequencies associated with local maxima of the frequency-domain PVP signal, including a heart rate frequency (F₁), a first harmonic frequency (F₂) at a first harmonic of the heart rate frequency, and a second harmonic frequency (F₃) at a second harmonic of the heart rate frequency. The evaluation unit calculates a blood volume metric based at least in part upon the heart rate frequency (F₁), the first harmonic frequency (F₂), and the second harmonic frequency (F₃).

In an aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the blood volume metric is an indication of one of the following: hypovolemia, hypervolemia, or euvolemia.

In an aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, obtaining the time-domain PVP signal includes receiving and storing the plurality of values of the electronic signal over the sample period.

In an aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the sample period is a fixed duration that is continuously updated on a rolling basis as new values of the electronic signal are obtained.

In an aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the transformation is a fast Fourier transformation.

In an aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the blood volume metric is further determined at least in part based upon at least one or more additional harmonic frequency associated with additional harmonics of the heart rate frequency (F₁).

In an aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the blood volume metric is determined based at least in part upon an increase or decrease in a magnitude of the frequency-domain PVP signal associated with the first harmonic frequency (F₂).

In an aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, a method of measuring and evaluating blood volume of a patient using a measurement associated with a peripheral venous pressure (PVP) within a peripheral vein of a circulatory system of the patient includes monitoring, by a transducer of a PVP sensor, a physical phenomenon associated with the PVP within the peripheral vein of the circulatory system of the patient. The method includes generating, by the PVP sensor, an electronic signal associated with the PVP over a sample period, the values of the electronic signal over the sample period forming a time-domain PVP signal. The method includes applying, by a processor of an evaluation unit, a transformation to the time-domain PVP signal to generate a frequency-domain PVP signal. The method includes identifying, by the processor of the evaluation unit, a plurality of frequencies associated with local maxima of the frequency-domain PVP signal, including a heart rate frequency (F₁), a first harmonic frequency (F₂) of the heart rate frequency, and a second harmonic frequency (F₃) of the heart rate frequency. The method includes calculating, by the processor of the evaluation unit, a blood volume metric based at least in part upon the heart rate frequency (F₁), the first harmonic frequency (F₂), and the second harmonic frequency (F₃).

In an aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the blood volume metric is an indication of one of the following: hypovolemia, hypervolemia, or euvolemia.

In an aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the plurality of frequencies further include one or more additional harmonic frequencies associated with additional harmonics of the heart rate frequency (F₁).

In an aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the one or more additional harmonic frequencies are identified as the frequencies associated with local maxima of the frequency-domain PVP signal nearest to integer multiples of the heart rate frequency (F₁).

In an aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the blood volume metric is further determined at least in part based upon at least one of the additional harmonic frequencies.

In an aspect of the present disclosure, which may be combined with any other aspect listed herein unless specified otherwise, the blood volume metric is determined based at least in part upon a harmonics ratio between the first harmonic frequency (F₂) and the at least one of the additional harmonic frequencies.

It is accordingly an advantage of the present disclosure to provide improved accuracy with respect to PIVA monitoring.

It is another advantage of the present disclosure to provide a PIVA monitoring devices, systems, and methods, which may more accurately monitor patient parameters.

It is yet another advantage of the present disclosure to provide a PIVA monitoring devices, systems, and methods, which may provide earlier warnings of potential problems with a patient.

It is still a further advantage of the present disclosure to provide a PIVA monitoring devices, systems, and methods, which may identify additional patient conditions.

Additional features and advantages of the disclosed devices, systems, and methods are described in, and will be apparent from, the following Detailed Description and the Figures. The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the figures and description. Also, any particular embodiment does not have to have all of the advantages listed herein. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.

BRIEF DESCRIPTION OF THE FIGURES

Understanding that the figures depict only typical embodiments and are not to be considered to be limiting the scope of the present disclosure, the present disclosure is described and explained with additional specificity and detail through the use of the accompanying figures. The figures are listed below.

FIG. 1 illustrates a block diagram of an exemplary PIVA system for use in measuring, analyzing, and responding to a patient's peripheral venous blood pressure, according to an example embodiment of the present disclosure.

FIG. 2 illustrates a block diagram of an exemplary PIVA device for implementing some functions of the exemplary PIVA system, according to an example embodiment of the present disclosure.

FIG. 3 illustrates a flow diagram of an exemplary PIVA measurement and analysis method for measuring and analyzing a patient's peripheral venous blood pressure, according to an example embodiment of the present disclosure.

FIG. 4A illustrates an exemplary plot of time-domain representation of a peripheral venous pressure signal, according to an example embodiment of the present disclosure.

FIG. 4B illustrates an exemplary plot of frequency-domain representation of a peripheral venous pressure signal, according to an example embodiment of the present disclosure.

FIG. 4C illustrates an exemplary plot of frequency-domain representation of a peripheral venous pressure signal, according to an example embodiment of the present disclosure.

FIG. 5 illustrates an exemplary PIVA comparison method for identifying changes in a patient status based upon comparison of peripheral venous pressure over time, according to an example embodiment of the present disclosure.

FIG. 6 illustrates a block diagram of an exemplary PIVA system for use in measuring, analyzing, and responding to a patient's peripheral venous blood pressure, according to an example embodiment of the present disclosure.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1 illustrates a block diagram of an exemplary PIVA system 100 for use in measuring, analyzing, and responding to peripheral venous blood pressure of a patient 102. The exemplary PIVA system 100 or a similar system may be used to implement the various techniques for monitoring patient status based upon measurements associated with PVP for the patient 102. The PIVA system 100 may measure a pressure signal associated with the patient's peripheral vein, analyze the pressure using PIVA techniques to identify key frequency components of the pressure signal, and analyze the key frequency components of the pressure signal to determine patient status based upon one or more metrics, as discussed below.

The PIVA system 100 includes an IV tube 104 in fluid connection with the circulatory system of the patient 102. Specifically, a venous access device 106 may be inserted into a peripheral vein 108 of the patient 102 at an access point. The venous access device 106 may include a needle, catheter, cannula, or other means of establishing a fluid connection between the IV tube 104 and the peripheral vein 108. The venous access device 106 may be a separate component connected to the IV tube 104 or may be formed as an integral portion of the IV tube 104. In either case, the venous access device 106 may include a terminal end inserted into the peripheral vein 108 at the access point and a connecting end that connects to a primary portion of the IV tube 104. The primary portion of the IV tube 104 may serve as a conduit between the venous access device 106 and a fluid source 110.

At some point along the primary portion of the IV tube 104, a pressure sensor 112 may be disposed in fluid connection with the interior of the IV tube 104. The pressure sensor 112 is thus also in fluid connection with the peripheral vein 108 of the patient through the IV tube 104 and the venous access device 106. The pressure sensor 112 is thereby enabled to measure pressure changes in the peripheral venous system of the patient 102 based upon changes in the fluid pressure within the IV tube 104. In other embodiments, the pressure sensor 112 may not be disposed in fluid connection with the interior of the IV tube 104. For example, the pressure sensor 112 may instead be attached to the exterior of the IV tube 104 and thereby not directly in fluid communication with the fluid source 110. The pressure sensor 112 may, in some such embodiments, measure pressure based upon acoustic or optical phenomenon at the sensor location. In some embodiments, the pressure sensor 112 may be disposed at a terminating end (i.e., a capped off end) of an IV tube 104 inserted specifically for the purpose of measuring pressure within the peripheral vein 108, in a manner similar to the device illustrated below in FIG. 2. In still other embodiments, other sensors may be used instead of the pressure sensor 112, such as force sensors, sound sensors, and sensors that need not be in fluid contact with the peripheral vein 108 of the patient through the IV tube 104 and the venous access device 106. In yet further embodiments, the sensor 112 may be disposed within a portion of a needle, catheter, or other venous access device 106 that is inserted within the peripheral vein 108 of the patient 106, such that PVP may be measured in situ within the peripheral vein 108. Such in situ measurement is advantageous inasmuch as it obviates the effect of temperature, viscosity, and other factors on transmission of pressure within the IV tube 104.

In various embodiments, the pressure sensor 112 may be positioned at various distances from the access point of the peripheral vein 108, from a location within the peripheral vein 108 or a location proximate to the connecting end of the venous access device 106 to a position proximate to the fluid source 110 or at a terminating end of the IV tube 104. The pressure sensor 112 is shown in FIG. 1 as being at an intermediate location along the length of the IV tube 104, in order to better illustrate the various components of the PIVA system 100. In some embodiments, the pressure sensor 112 may directly measure fluid pressure within the IV tube 104. Specifically, the pressure sensor 112 may include a transducer that provides an electronic pressure signal indicative of the pressure detected by the transducer to an analysis component 114 via a connection 122. The electronic pressure signal may be an analog electrical signal directly provided by the transducer or may be a preprocessed digital signal indicating pressure values based upon the transducer interface with the primary portion of the IV tube 104. In embodiments in which the pressure sensor 112 is not in fluid connection with the IV tube 104 or the peripheral vein 108, the pressure sensor 112 may nonetheless include one or more transducers to generate electronic signals associated with the PVP. For example, the pressure sensor 112 may use one or more microphones disposed to detect sound at an exterior surface of an IV tube 104 to generate electronic pressure signals indicative of pressure within the IV tube 104 as a proxy for PVP within the peripheral vein 108.

The analysis component 114 is communicatively connected to the pressure sensor 112 to receive the electronic pressure signal via the connection 122. The analysis component 114 may include general-purpose or special-purpose processing hardware, such as microprocessors or special-purpose analysis circuits. As shown, the analysis component 114 may include one or more units for performing the PIVA analysis. A response unit 116 may identify and control responses based upon the pressure data from the pressure sensor 112. The response unit 116 may control the presentation of alarms, such as visual alarms (e.g., via monitor 120) and audible alarms (e.g., via a speaker in communication with the PIVA system 100). Likewise, the response unit 116 may control the operation of the fluid source 110, such as by controlling the rate of fluid flow. To determine appropriate responses, the response unit 116 may receive evaluation data from an evaluation unit 118, which may include metrics determined from the electronic pressure signal. The evaluation unit 118 may obtain pressure values (or signal values directly or indirectly associated with PVP) from the electronic pressure signal and evaluate the pressure values to determine information regarding the patient 102, such as blood volume metrics or other metrics as described in further detail below. The information generated by the evaluation unit 118 may also be stored or presented for patient monitoring (e.g., via monitor 120). In alternative embodiments, additional, fewer, or alternative units may be included. For example, the evaluation unit 118 may perform the functions ascribed to the response unit 116 herein.

The analysis component 114 may be communicatively connected to a monitor 120 via a connection 126 in some embodiments. The monitor 120 may be a separate monitor for displaying information regarding the patient or may be incorporated into another device, such as a pump or other fluid source device. The monitor 120 may also be communicatively connected to the fluid source 110 via a connection 128 to receive and display information associated with the fluid source 110. In some embodiments, the monitor 120 may be used to control the operation of the fluid source 110, such as by adjusting fluid flow rate, duration of operation, mode of operation, etc. The analysis component 114 may similarly be communicatively connected to the fluid source 110 via connection 124 in some embodiments. The analysis component 114 may receive information regarding operation of the fluid source 110 for use in evaluating the patient by the evaluation unit 118. The response unit 116 may also communicate with the fluid source 110 to control operation of the fluid source 110 in response to information regarding the patient determined based upon the electronic pressure signal from the pressure sensor 112.

The various connections 122, 124, 126, and 128 may each be wired or wireless connections in various embodiments. Moreover, some or all of the connections 122, 124, 126, and 128 may be internal to devices, such as a PIVA device 130 or a PIVA-integrated fluid source 140.

The PIVA device 130 may incorporate the pressure sensor 112 and analysis component 114 (along with associated connections) into a device that may be attached to or within the IV tube 104 to perform PIVA monitoring of the patient 102. The PIVA-integrated fluid source 140 may include a computer-controlled fluid reservoir or pump configured to utilize PIVA monitoring of the patient 102 in controlling fluid flow. Like the PIVA device 130, the PIVA-integrated fluid source 140 may include the pressure sensor 112 and analysis component 114, along with the fluid source 110 and the monitor 120 (along with associated connections). Alternative embodiments may include additional, fewer, or alternative components in alternative configurations.

FIG. 2 illustrates a block diagram of an exemplary PIVA device 130 for implementing some functions of the exemplary PIVA system 100. As illustrated, the exemplary PIVA device 130 may be configured to attach to a spur 104A of the IV tube 104, such as at one branch of a Y-connector or a T-connector. As above, the PIVA device 130 may include a pressure sensor 112 disposed such that a sensing portion is in contact with fluid in the IV tube 104 (illustrated as the IV tube spur 104A). The pressure sensor 112 may be electrically communicatively connected to a microprocessor 132 via a system bus 138. The microprocessor 132 (MP) may be further communicatively connected to a program memory 134 and a communication unit 136 (COMM UNIT) via the system bus 138. The program memory 134 may be a non-transitory, non-volatile memory (e.g., a flash memory) storing executable instructions that may be executed by the microprocessor 132 to evaluate the electronic pressure signal from the pressure sensor 112, determine patient information (e.g., blood volume metrics and/or respiratory metrics), determine appropriate responses to the determined patient information, and control the communication unit 136 to electronically communicate with the fluid source 110 or monitor 120 via connections 124 or 126. The program memory 134 may store a plurality of routines, scripts, or modules corresponding to units or sub-units of the analysis component 114. The communication unit 136 may be a hardware component configured to send and receive electronic data between the PIVA device 130 and the fluid source 110 or monitor 120 via connections 124 or 126. The connections 124 and 126 are illustrated as being wired connections in the exemplary PIVA device 130, which may also be used to obtain power for the PIVA device 130. Alternatively, another power connection or battery (not shown) may provide power to the PIVA device 130.

Although the PIVA device 130 is illustrated as being connected to a spur 140A of the IV tube 104, some embodiments may include an IV tube 104 terminating at the spur 104A, without another branch or portion of the IV tube 104 that connects to a fluid source 110. In such embodiments, the entire IV tube 104 may consist of the spur 104A as a non-branching tube, which may directly connect to or incorporate the venous access device 106. Thus, the PIVA device 130 may cap a terminating portion of such an IV tube 104/104A, such that no fluid source 110 is connected to the peripheral vein 108 through the same IV tube 104/104A and venous access device 106. In this embodiment, a fluid source could be otherwise connected to provide fluids to the patient 102 via another IV tube and another venous access device.

FIG. 3 illustrates a flow diagram of an exemplary PIVA measurement and analysis method 300 for measuring and analyzing a status of a patient 102 based on PVP using the PIVA system 100. The method 300 may be used to determine various patient status metrics, such as metrics related to patient blood pressure, blood volume, respiration, respiratory volume, respiratory effort, or systemic vascular resistance. The method 300 may be performed by the evaluation unit 118 using an electronic pressure signal from the pressure sensor 112, the generation of which electronic pressure signal by the pressure sensor 112 may be included in the method 300 in some embodiments.

The method 300 begins with measuring a PVP data signal for the patient 102 (block 302). The PVP data signal may be measured by using a transducer of the pressure sensor 112 to generate an electronic pressure signal associated with the pressure within the IV tube 104. Because the IV tube 104 is in fluid connection with the peripheral vein 108 of the patient 102 via the venous access device 106, the pressure in the IV tube 104 measured by the pressure sensor 112 is associated with the pressure in the peripheral vein 108. In some embodiments of the PIVA system 100, the pressure within the IV tube 104 may be different from the PVP within the peripheral vein 108, but the pressure measured within the IV tube 104 may nonetheless be proportional to, or otherwise related to, the PVP in the peripheral vein 108. Thus, the measured PVP data signal may be adjusted to compensate for differences between the pressures, if desired. For example, adjustments may be made based upon temperature, viscosity of the patient's blood or a fluid provided by the fluid source 110, or a gauge or rigidity of the IV tube 104, or other system variation that may impact the PIVA signal. Whether adjusted or unadjusted, the PVP data signal measured by the pressure sensor 112 accurately represents changes in pressure over time, including both periodic pressure changes associated with respiratory and circulatory cycles and aperiodic pressure changes that may be indicative of changes in patient condition. Similarly, a PVP data signal generated by the pressure sensor 112 by components not in fluid contact with the interior of the IV tube 104, likewise, provides a representation of the pressure within the peripheral vein 108 of the patient 102. The PVP data signal may be the electronic pressure signal generated by the pressure sensor 112 or may be a data signal derived therefrom. In alternative embodiments, the PVP data signal may be evaluated in real-time as it is generated, or it may be stored for later analysis.

FIG. 4A illustrates an exemplary chart of a time-domain representation of the PVP data signal, which may be the electronic pressure signal from the pressure sensor 112. The chart illustrates a time-domain PVP signal 402, which shows periodic increases and decreases in pressure associated with the patient heartbeat. Additionally, the time-domain PVP signal 402 exhibits slower cyclical variation as a result of, for example, patient respiration. The chart also illustrates a respiration curve 404 that shows the effect of inspiration and expiration on the time-domain PVP signal 402. For example, because of the expansion of the lungs during inspiration, the baseline measured venous blood pressure is higher during inspiration than during expiration, when the volume of the lungs is reduced due to the expansion of the diaphragm pressing against the flexible vena cava (and other large veins) in the abdominal cavity and/or by increasing the peripheral venous flows toward the heart. On the other hand, expiration relieves the pressure caused by the diaphragm proximation against the vena cava. Many other factors influence PVP, such as blood volume and patient movement.

The time-domain PVP signal 402 is thus a combination of a plurality of influences, both periodic (e.g., heart rate or respiration) and aperiodic (e.g., movement or blood loss). Because the resulting time-domain PVP signal 402 will include noise from various sources, it may be difficult to detect small changes in pressure that may serve as indications of patient status. Therefore, PIVA techniques utilize a frequency-domain evaluation of the PVP data signal, as described below. It should be recognized that, although the time-domain representation of the PVP data signal is illustrated graphically as a chart in FIG. 4A to illustrate the salient features of the data, it is not necessary to produce a chart or other graphical representation of such data signal. Instead, in certain embodiments, the PVP data signal is processed by the evaluation unit 118 without generating a graphical representation of the time-domain PVP data signal.

Returning to FIG. 3, a plurality of data values may then be obtained from the measured PVP data signal (block 304). The evaluation unit 118 may sample values of the live or stored PVP data signal to obtain the plurality of data values. In some embodiments, the data values may be sampled at fixed intervals over a period of time to obtain a plurality of data values within an evaluation window, which may include storing the plurality of data values associated with the window in temporary or permanent electronic data storage. In further embodiments, data for multiple evaluation windows may be obtained, such that each evaluation window includes a plurality of data values. For example, concurrent time periods may be identified as separate evaluation windows, or evaluation windows may be identified as time periods separated by an intervening period (e.g., twenty-second evaluation windows beginning every minute, thus separated by forty-second intervening periods). When the evaluation unit 118 samples values of a live (continuously updating) PVP data signal, in some embodiments, the evaluation window may be updated on a rolling basis to obtain new data values while covering time periods of fixed duration. For example, the evaluation window may be repeatedly updated by adding new sample data values and removing the oldest sample data values to maintain a window of a fixed duration (e.g., five seconds, ten seconds, twenty seconds, etc.) of the most recent PVP data from the pressure sensor 112. Where the evaluation unit 118 periodically obtains updates of new sample data values, the window may be updated (and the transformation and evaluation described below may be performed for the updated window) every time a new data value is received. In an alternative embodiment, the plurality of data values may correspond to the continuous values of an analog PVP data signal, which may be obtained and analyzed by analog electronic equipment (which may be part of the evaluation unit 118).

From the plurality of data values, the evaluation unit 118 generates frequency-domain data corresponding to the plurality of data values (block 306). This may include applying a data transformation to the plurality of data values representing a time-domain PVP signal to produce a frequency-domain representation of the PVP signal. In a preferred embodiment, the evaluation unit 118 applies a fast Fourier transform (FFT) to the sampled plurality of data values. The FFT may be applied periodically (e.g., every ten seconds, every minute, or every two seconds, with or without overlapping evaluation windows). Other analysis techniques that can separate frequencies and identify local maxima according to frequency are contemplated, such as wavelet transform.

The frequency-domain data may include a plurality of values representing the magnitude of various frequency components in the measured PVP data signal based upon the plurality of data values. Such values may be discrete or may be part of a curve of magnitudes corresponding to frequencies, which curve may be generated by interpolation or approximation between a finite number of values associated with a finite number of frequencies. Although FFT algorithms may be used to great effect, other time-frequency transforms or other techniques of analyzing frequency components of signals may be utilized to evaluate the plurality of data values. For example, in addition to other Fourier transforms, the evaluation may include wavelet transforms or time-frequency representations of the measured PVP data signal.

FIG. 4B illustrates an exemplary chart of a frequency-domain representation of the PVP data signal, corresponding to the time-domain PVP signal 402 represented in the time domain in FIG. 4A. The chart illustrates the magnitude of each frequency component by a frequency curve 406. Because the chart in FIG. 4B is a frequency-domain representation, the horizontal axis represents frequency, and the vertical axis represents magnitude. Although the chart is exemplary, certain typical features may be discerned therein. Of particular interest are the several peaks (P_(N)) of the frequency curve 406 associated with frequencies (F_(N)). Between the peaks, minor variations in magnitude are seen, which may represent minor components of the time-domain PVP signal 402 associated with noise in the system or artifacts of the circulatory system (e.g., movements of the patient during measurement, openings and closings of the atrioventricular and aortic valves, etc.). In certain embodiments, the frequency domain representation generated by PIVA system 100 is displayed (e.g., via monitor 120).

Although the frequency-domain representation of the PVP data signal is illustrated in FIG. 4B as a chart to illustrate the salient features, it should be understood that it is not necessary to produce a chart or other graphical representation of the frequency-domain data. Indeed, in certain embodiments, no such graphical representation is generated. Instead, the frequency-domain data is processed by the evaluation unit 118 as an intermediate process, the results of which are not directly presented to a user of the system or device.

Under ordinary conditions, the peak (P₀) with the lowest frequency (F₀) corresponds to the respiration rate of the patient 102, and the peak (P₁) with the next-lowest frequency (F₁) corresponds to the heart rate of the patient 102. One or more harmonic peaks (P_(H)) associated with harmonic frequencies (F_(H)) of the heart rate frequency (F₁) may be identified in some embodiments. Such harmonic peaks (P_(H)) are associated with local maxima of the frequency curve 406. For example, the next two peaks (P₂) and (P₃) of the frequency curve 406 are harmonic peaks (P_(H)) occurring at frequencies associated with the first and second harmonics of the heart rate at the first harmonic frequency (F₂) and the second harmonic frequency (F₃) respectively. The identified harmonics occur at fixed multiples of the heart rate frequency (F₁). Typically, these multiples are integer multiples. Specifically, experimental data indicate that first harmonic frequency (F₂) is approximately twice the heart rate frequency (F₁), and the second harmonic frequency (F₃) is approximately thrice the heart rate frequency (F₁), as illustrated by FIG. 4B.

Identification of the peaks (e.g., P₁, P₂, P₃) of the corresponding frequencies (e.g., F₁, F₂, F₃), such as via the evaluation unit 118, provides for subsequent calculations of patient status (e.g., hemodynamic status). For example, the peaks (e.g., P₁, P₂, P₃) of the corresponding frequencies (e.g., F₁, F₂, F₃) may be used to calculate a PIVA Score, as further detailed herein.

Although not shown, additional peaks associated with third and higher harmonics of the heart rate may be identified in some embodiments. The further harmonic frequencies (F₄, F₅, . . . F_(N)) typically occur at corresponding sequential integer multiples of the heart rate frequency (F₁). For example, a second harmonic frequency may be represented by F₃, a third harmonic frequency may be represented by F₄, etc. Although some variation exists in the observed frequencies of the peaks associated with the harmonic frequencies, the harmonic frequency peaks have been found to occur at frequencies within a range of ten percent (i.e., ±10%) of the value of the heart rate frequency above or below the integer multiples of the heart rate frequency (F₁). The relationships between the magnitudes of the peaks (P_(N)) may vary, but typically the magnitude of the peak (P₁) associated with the heart rate frequency (F₁) should be greater than the magnitudes of the peaks (P₂), (P₃), etc., associated with the harmonic frequencies (F₂), (F₃), etc., thereof.

Furthermore, it should be noted that while FIG. 4B illustrates the frequency curve 406 as a number of parabolic peaks (e.g., P₀, P₁, P₂, P₃), other graphical representations of the frequency-domain representation should be expected. For example, to the extent that the system is consistent (e.g., consistent patient respiration and heart rate), and the sampling rate is high enough (e.g., the sampling rate of data values measured in the time domain), the peaks (e.g., P₀, P₁, P₂, P₃) may be depicted graphically as vertical lines (e.g., parabolic peaks with unperceivable width or parabolic peaks with no width).

Although the present disclosure generally refers to the respiration rate as corresponding to the lowest-frequency peak (P₀), the heart rate as corresponding to the next-lowest frequency peak (P₁), and so on, it should be appreciated that any such reference is done for ease of explanation. To this end, in some embodiments, the time-domain PVP signal may detect one or more frequencies lower than the respiration rate. For instance, gut frequencies tend to be associated with lower frequencies than a typical respiratory frequency. In these embodiments, the peak (P₀) with the lowest frequency (F₀) corresponds to a gut frequency, and the peak (P₁) with the second-lowest frequency (F₁) corresponds to the respiratory frequency. Similarly, the heart rate frequency and each of the corresponding harmonic frequencies would correspond to the next-lowest peak (P₂) and the following peaks (P₃, P₄, . . . P_(N)), respectively. It should be appreciated that in some further embodiments, the time-domain PVP signal may detect multiple frequencies lower than the respiratory frequency. Accordingly, the peak index corresponding to the respiration rate, the heart rate, and the heart rate harmonics may increase by the number of frequencies detected lower than the respiration rate. As such, unless specifically described otherwise, any reference to the respiratory rate corresponding to the lowest frequency peak (P₀) and the heart rate frequency corresponding to the next-lowest frequency peak (P₁) is not limiting and also envisions offsetting the correspond peak indexes by the number of lower-than-respiration rate frequencies detected by the time-domain PVP signal.

Turning again to FIG. 3, the evaluation unit 118 further identifies a plurality of peaks (P_(N)) corresponding to frequencies (F_(N)) of the frequency-domain representation of the PVP signal (block 308), such as the frequency curve 406. The evaluation unit 118 may first identify values indicating peaks (P_(N)) in the frequency-domain representation of the PVP signal by comparison of the frequency-domain PVP signal values, then identify the corresponding frequencies (F_(N)) associated with the identified peak values (P_(N)). To determine the peak values (P_(N)), the evaluation unit 118 may utilize any of various methods to identify local maxima as peaks, including methods based upon any or all of a comparison of the relative magnitudes of local maxima, establishment of fixed or dynamic frequency bands around each peak, or comparison of full width at half maximum for local maxima. For example, a band-pass filter may be employed to separate segments of the frequency-domain representation of the PVP signal to further identify local maxima. This may be particularly useful in identifying harmonic peaks (P_(N)) and corresponding harmonic frequencies (F_(H)) because such harmonics occur at integer multiples of the heart rate frequency (F₁).

As an example, a band-pass filter centered around a frequency twice the heart rate frequency (F₁) and having a band width of twenty percent of the heart rate frequency (F₁) may be used to define a range of the frequency-domain representation of the PVP signal that contains the first harmonic peak (P₂). The first harmonic frequency (F₂) may then be identified by simply determining the frequency associated with the local maximum value of the frequency-domain representation of the PVP signal within such range. By employing these or other techniques, the peaks (P_(N)) of the frequency-domain representation of the PVP signal may be distinguished from other local maxima arising from noise or other minor phenomena in the circulatory system.

Once the plurality of peaks (P_(N)) associated with the frequencies (F_(N)) have been identified, the evaluation unit 118 may analyze the magnitudes of the frequency-domain representation of the PVP signal at one or more of the frequencies (F_(N)) to determine one or more aspects of patient status (block 310). Such analysis may include determining one or more patient status metrics, such as a blood volume metric, respiratory volume metric, respiratory effort metric, systemic vascular resistance metric, other metric relating to the systemic vascular resistance (e.g., mean arterial pressure, mean venous pressure, cardiac output), or the like for the patient 102. For example, the patient status metrics may include a blood volume metric indicating one of the following hemodynamic states of the patient 102: hypovolemia, hypervolemia, or euvolemia. In certain embodiments, hemodynamic states of the patient 102 may be determined as a score or as a category of patient status.

Some patient status metrics may be determined directly from the one or more frequencies (F_(N)) or magnitudes of the frequency-domain representation of the PVP signal associated therewith. For example, respiratory rate may be determined based upon the respiratory frequency (F₀). Likewise, for example, respiratory depth may be determined based upon the magnitude associated with the respiratory frequency (F₀) (i.e., the magnitude of the respiratory peak (P₀)), or a blood volume metric may be determined based upon the magnitude associated with the heart rate frequency (F₁) (i.e., the magnitude of the heart rate peak (P₁)). As another example, a blood volume metric indicative of patient hemodynamic state (e.g., hypovolemia or hypervolemia) may be directly measured or calculated.

For example, as previously mentioned, subsequent to performing transformation, the evaluation unit 118 may identify the peaks (e.g., P₁, P₂, P₃) of the corresponding frequencies (e.g., F₁, F₂, F₃). These individual peaks (e.g., P₁, P₂, P₃) corresponding to the various frequencies, such as the heart rate frequency F₁, the first harmonic of the heart rate frequency F₂, and the second harmonic of the heart rate frequency F₃ may then be used in an equation to calculate a PIVA Score. PIVA Score, representative of a patient's fluid status, is also a corollary for pulmonary capillary wedge pressure. Because pulmonary capillary wedge pressure is an indicator of fluid status (e.g., hypervolemia or hypovolemia), the PIVA Score, likewise, is representative of a patient's fluid status.

In an embodiment, the equation to calculate PIVA Score is represented by the following:

${{PIVA}\mspace{14mu} {Score}} = {{c_{3}{\tanh \left( \frac{g_{0} + {g_{1}{mag}^{f\; 1}} + {g_{2}{mag}^{f\; 2}} + {g_{3}{mag}^{f\; 3}}}{2} \right)}} + {c_{2}{\tanh \left( \frac{h_{0} + {h_{1}{mag}^{f\; 1}} + {h_{2}{mag}^{f\; 2}} + {h_{3}{mag}^{f\; 3}}}{2} \right)}} + {c_{1}{\tanh \left( \frac{i_{0} + {i_{1}{mag}^{f\; 1}} + {i_{2}{mag}^{f\; 2}} + {i_{3}{mag}^{f\; 3}}}{2} \right)}} + c_{0}}$

Each of c₀, c₁, c₂, c₃, g₀, g₁, g₂, g₃, h₀, h₁, h₂, h₃, i₀, i₁, i₂, and i₃ are constants. Each of mag^(f1), mag^(f2), and mag^(f3) represents the individual magnitudes of each of the respective frequencies (e.g., F₁, F₂, F₃). These magnitudes are also commonly referred to herein as peaks of frequencies. For example, mag^(f1) may also be referred to as peak P₁ herein, associated with heart rate frequency F₁. Similarly, for example, mag^(f2) may also be referred to as peak P₂ herein, associated with first harmonic frequency F₂. Similarly, for example, mag^(f3) may also be referred to as peak P₃ herein, associated with second harmonic frequency F₃. For example, and with reference to FIG. 4B, P₁, referred to in the PIVA Equation as mag^(f1), is the magnitude of the heart rate frequency (F₁), P₂, referred to in the PIVA Equation as mag^(f2), is the magnitude of a first harmonic frequency (F₂), and P₃, referred to in the PIVA Equation as mag^(f3), is the magnitude of a first harmonic frequency (F₃)

Evaluation unit 118 calculates the PIVA score, which is unitless. In a related embodiment, PIVA system 100 displays the PIVA score (e.g., via monitor 120). By calculating PIVA score, a patient's fluid status may be readily determined (e.g., hypovolemia, hypervolemia, or euvolemia). Preferably, the calculated PIVA score has an agreement with pulmonary capillary wedge pressure of ±8 mmHg with limits of agreement of 95% confidence interval.

In an embodiment, additional peak magnitudes corresponding to various frequencies (e.g., P₄ corresponding to F₄, a third harmonic frequency) may also be used in calculating the PIVA score (e.g., implementing additional constants as well) for greater accuracy in calculation.

FIG. 5 illustrates an exemplary PIVA comparison method 500 for identifying changes in a patient status based upon comparison of frequency-domain representations of PVP signals associated with different times. The PIVA comparison method 500 may be implemented by the evaluation unit 118 and the response unit 116 to determine and respond to changes in patient status between time periods. For example, the evaluation unit 118 may determine and compare frequency-domain representations of PVP based upon electronic pressure signals received during multiple time periods to determine changes in patient metrics, such as blood pressure, blood volume, respiratory volume, respiratory effort, or systemic vascular resistance. Specifically, the evaluation unit 118 may compare relative or absolute magnitudes associated with frequencies (F_(N)) of peaks (P_(N)) in the frequency distributions determined for each time period to identify changes in patient status that may be used by the response unit 118 to determine and implement response actions.

The exemplary method 500 begins by obtaining a first frequency distribution associated with a first time period (block 502) and a second frequency distribution associated with a second time period (block 504). Each of the first and second frequency distributions may be generated as the frequency-domain data corresponding to the plurality of data values from the PVP data signal by the method 300, as described above. The first and second time periods may correspond to first and second evaluation windows, as discussed above, each evaluation window being associated with a plurality of data values sampled or received by the evaluation unit 118. The data values for each of the first and second evaluation windows may be stored in a volatile or non-volatile memory until needed by the evaluation unit 118 to generate the frequency distributions, as discussed above. Alternatively, the frequency distributions or information associated therewith (e.g., frequency peaks and associated magnitudes) may be stored directly for comparison. In some embodiments, the first and second frequency distributions may be frequency-domain representation of the PVP signals from the sensor 112 for time periods of fixed duration beginning at times separated by a predetermined interval. For example, the method 500 may be implemented on a rolling basis (i.e., periodically or when new PVP data becomes available) during real-time monitoring of the patient 102 by comparing the magnitudes of frequency peaks (F_(N)) of frequency distributions generated for first and second evaluation windows during patient monitoring. The first and second periods may be partially overlapping, adjacent in time, or separated by an intervening period.

The evaluation unit 118 may next identify one or more peaks of interest for determining the patient status (block 506). In a related embodiment, the evaluation unit 118 receives an indication of one or more peaks of interest (e.g., a user request for a peak of interest). In one embodiment, the evaluation unit 118 may receive the indication from an input device (e.g., a keyboard or a touchscreen), a portable electronic device (e.g., a mobile phone or a tablet), the PIVA device 130, or a non-PIVA circulatory pressure monitoring device.

The peaks of interest may be identified in either one or both of the first and second frequency distributions. In some instances, the one or more peaks of interest may be determined based upon peaks (P_(N)) in a baseline frequency distribution generated for the patient 102, which may be the first frequency distribution or an additional prior frequency distribution. The baseline frequency distribution may, for example, be determined prior to scheduled surgery to establish a baseline for later patient status monitoring. The peaks of interest may be identified based upon the associated frequencies (F_(N)), such as by identifying the respiratory frequency (F₀) or the heart rate frequency (F₁). In some embodiments, the peaks of interests may include a plurality of such peaks, such as the peaks (P₂) and (P₃) associated with the first harmonic frequency (F₂) and the second harmonic frequency (F₃). Under some conditions, not all peaks of interest may be identifiable in both frequency distributions. For example, during an acute failure of the circulatory system, systemic vascular resistance may markedly decrease and peaks associated with the harmonic frequencies (F₂, F₃, . . . F_(N)) may not be discernible. Thus, the peaks associated with the harmonic frequencies (F₂, F₃, . . . F_(N)) may be identifiable in the first frequency distribution but not in the second frequency distribution. Nonetheless, a change in magnitude of the frequency distributions at the harmonic frequencies (F₂, F₃, . . . F_(N)) may be determined by comparison of the first and second frequency distributions.

Based upon the identified one or more peaks of interest, the evaluation unit 118 may further determine a patient status (or a change in patient status) by a comparison of the first and second frequency distributions (block 508). Determining the patient status may include a comparison of the magnitudes associated with the same one or more frequencies (F_(N)) between the first and second frequency distributions, a comparison of the values of a function of a plurality of magnitudes associated with frequencies between the first and second frequency distributions (e.g., a comparison of ratios of peak magnitudes), a comparison of frequencies (F_(N)) associated with one or more peaks (P_(N)) between the first and second frequency distributions (e.g., a change in the respiratory frequency or heart rate frequency), or a comparison of other metrics associated with patient status. In some embodiments, the patient status may be determined based upon a change in a metric beyond a threshold level. For example, a decrease in the magnitude associated with the heart rate frequency (F₁) in the second frequency distribution below 80% of the corresponding magnitude associated with the heart rate frequency (F₁) in the first frequency distribution may indicate hypovolemia in the patient 102. As another example, a decrease in the ratio of the magnitude associated with the first harmonic frequency (F₂) to the magnitude associated with the heart rate frequency (F₁) between the first and second frequency distributions beyond a predetermined threshold may indicate hypervolemia or hypovolemia, depending upon whether and how the magnitude associated with the heart rate frequency (F₁) changes. Comparisons of particular interest are discussed in further detail elsewhere herein.

A comparison involving one or more of the harmonic frequencies (F₂, F₃, etc.) is of particular interest regarding patient hemodynamic state or blood volume. Because the frequency distribution values associated with harmonic frequencies (F₂, F₃, etc.) are more sensitive to changes in blood volume than the values associated with the heart rate frequency (F₁), monitoring changes in the values associated with harmonic frequencies (F₂, F₃, etc.) may provide an earlier or clearer indication of patient hemodynamic state. For example, a sharp increase or decrease in the magnitude of the value of the frequency distribution associated with the first harmonic frequency (F₂) (or other harmonic frequency, such as the second harmonic frequency F₃) may be more pronounced than the corresponding change in the values associated with the heart rate frequency (F₁) in the same patient at the same time. In one embodiment, comparison of an increase or decrease associated with the first harmonic frequency (F₂) involves comparison of two separately calculated PIVA scores, such that changes to both the heart rate frequency (F₁) and the first harmonic frequency (F₂) are included (e.g., included via the PIVA score equation above).

Thus, blood volume metrics may be generated using the harmonic frequencies (F₂, F₃, etc.). Such metrics may be determined as functions of the harmonic frequencies (F₂, F₃, etc.), ratios of the frequency values of the harmonic frequencies (F₂, F₃, etc.), magnitudes associated with the harmonic frequencies (F₂, F₃, etc.), ratios of the magnitudes associated with the harmonic frequencies (F₂, F₃, etc.), or changes in any of these. Such changes may be measured against a baseline or against a previously determined value at a fixed interval in time prior to the current values. In some embodiments, the frequencies or magnitudes associated with the harmonic frequencies (F₂, F₃, etc.) may be compared against other relevant values, such as frequencies or magnitudes associated with the respiratory rate frequency (F₀) or the heart rate frequency (F₁). For example, one or more harmonic frequencies (F₂, F₃, etc.) may be normalized by comparison against the heart rate frequency (F₁). Such normalized value may be determined as a ratio of the magnitudes and may be used as a blood volume metric to evaluate the hemodynamic state of the patient 102. Other similar blood volume metrics based at least in part upon the frequency and magnitude values of the one or more harmonic frequencies (F₂, F₃, etc.) may be determined and used to evaluate the hemodynamic state of the patient 102 in various embodiments.

Once patient status has been determined, the response unit 116 may determine whether a response is required and cause any required response to be implemented (block 510). Additionally, or alternatively, the evaluation unit 118 or the response unit 116 may cause an indicator of the determined patient status to be stored or presented via the monitor 120 (block 510). If the response unit 116 determines a response is required, the response unit 116 may further determine one or more responses that are appropriate to address the identified patient status. Such responses may include generating an alarm or other warning that the patient status is abnormal, which may include information regarding the patient condition. An alarm or warning may be presented via the monitor 120 or may be communicated to another device for presentation. The alarm or warning may include a recommendation of one or more actions to take in response to the patient status. For example, the recommendation may include an adjustment to a fluid therapy for the patient 102, which may include a recommendation to administer one or more vasopressors or vasodilators. Such recommendation may be determined by the response unit 116 as part of the required response. In some embodiments, this may include sending an electronic communication to a user device (e.g., a workstation or mobile device used by a physician, nurse, or technician to monitor patient condition). In other embodiments, the alarm may include an audible alarm or notification (e.g., via a speaker).

The responses may similarly include controlling the fluid source 110 to adjust fluid flow to the patient 102. The fluid source 110 may be controlled to increase or reduce the rate of fluid flow to the patient 102, including starting or stopping fluid flow. In some embodiments, the response may include controlling the fluid source 110 (or a device connected thereto) to administer one or more drugs to the patient 102. For example, the fluid source 110 may be controlled to administer one or more vasopressors or vasodilators in a fluid delivered to the peripheral vein 108 via the IV tube 104 and venous access device 106. Where the fluid source 110 includes a pump, the response may include controlling the operation of the pump, such as by increasing or decreasing pump speed, flow rate, or mode of operation, as well as starting or stopping the pump. In some embodiments, the fluid source 110 may be controlled to administer a quantity of a drug to the patient 102 via the fluid. For example, the fluid source 110 may be controlled to add a quantity of the drug to the fluid. In some embodiments, the response may include controlling the rate of respirations associated with a ventilator and/or administering a medicament, such as albuterol, via a nebulizer.

Additional embodiments of specific analysis and response methods utilizing the PIVA system 100 are further described elsewhere herein in greater detail.

Magnitude Ratio and Other Comparisons

In some embodiments, the patient status metric may be determined based upon a comparison of magnitudes (P_(N) and P_(M)) associated with different frequency peaks (F_(N) and F_(M)) based upon the same plurality of data values (i.e., for the same evaluation window). For example, a ratio of the magnitudes associated with heart rate and first harmonic frequencies F₁ and F₂ may be used to determine a hemodynamic metric for the patient, such as a systemic vascular resistance or a blood volume status. Such ratios may be particularly useful in normalizing magnitudes associated with harmonic frequencies (F_(H)) to obtain more robust and more accurate patient status metrics. Similarly, ratios between magnitudes of the frequency-domain representation of the PVP signal associated with different harmonic frequencies (e.g., F₂ and F₃) may be used to determine hemodynamic state of the patient 102 (e.g., blood volume status). In further embodiments, the patient status metric may be determined based upon a comparison of magnitudes associated with the same one or more frequencies (F_(N)) of peaks (P_(N)) determined for different pluralities of data values (i.e., for different evaluation windows). For example, analysis of a change in the absolute or relative magnitude associated with heart rate frequency F₁ over time may be used to determine a hemodynamic metric. Information regarding the patient status may be stored in a memory, presented to a user via the monitor 120, or used by the response unit 116 to generate and implement a response (e.g., presenting an alarm or controlling the operation of the fluid source 110), including any of the responses discussed further below.

As mentioned above, a ratio of the magnitudes associated with frequency peaks F₁ and F₂ may be used to determine systemic vascular resistance. However, other ways to determine systemic vascular resistance are contemplated. For example, a change of one of the plurality of frequency peaks (e.g., F₁) or a change of one of the plurality of frequency peaks (e.g., F₁) relative to another of the plurality of frequency peaks (e.g., F₂) may be used to determine systemic vascular resistance. As another example, a difference between any two of the respiratory frequency (F₀), the heart rate frequency (F₁), the first harmonic frequency (F₂), and the second harmonic frequency (F₃) may be used to determine systemic vascular resistance. As another example, a ratio of any two of the respiratory frequency (F₀), the heart rate frequency (F₁), the first harmonic frequency (F₂), and the second harmonic frequency (F₃) may be used to determine systemic vascular resistance. If the determined systemic vascular resistance is abnormal, the fluid source 110 can provide fluid therapy to increase or decrease the systemic vascular resistance when the value is low or high, respectively. For example, when the response unit 116 generates and implements a response (e.g., controlling the operation of the fluid source 110), the systemic vascular resistance may be updated based at least in part upon a change in frequency peak of at least one of the respiratory frequency (F₀), the heart rate frequency (F₁), the first harmonic frequency (F₂), and the second harmonic frequency (F₃) in response to the fluid provided from the fluid source 110 to the circulatory system of the patient via the IV tube 104. The changes in frequency peak(s) in response to fluids may be able to differentiate between regular (hemorrhagic) shock and distributive (septic) shock. For instance, if the frequency-domain representation of the PVP signal of the patient returns to normal after fluid therapy, the patient is most likely in regular shock due to fluid loss, but if the FFT signal does not return to normal after a period, the patient may be in in distributive shock. Other forms of therapy for adjusting the systemic vascular resistance are contemplated, such as vasopressin and vasodilator medications.

Distinguishing Respiratory Peaks from Cardiac Peaks

In certain instances, similarities shared by respiratory and pulse rate waveforms pose risks of potential signal mis-identification, particularly when one or both of the respiratory signal or pulse rate signal fluctuates in magnitude. The relative x-axis location of the P₀ and P₁ peaks may be an unreliable reference for a practitioner to distinguish the respiratory signal from the pulse rate signal, particularly if a change in magnitude of one or both of the respiratory or pulse rate signal (e.g., a fluctuation) occurs while the practitioner's attention to the signal display was diverted.

As previously mentioned, an approach to interpreting signals on a PIVA display is to assume that the lower frequency slower peak (F₀) is the respiratory peak, and the next-faster (higher frequency) peak (F₁) represents the heart rate. For example, with reference to FIG. 4B and as previously noted above, under ordinary conditions, the peak (P₀) with the lowest frequency (F₀) corresponds to the respiration rate of the patient 102, and the peak (P₁) with the next-lowest frequency (F₁) corresponds to the heart rate of the patient 102.

This approach is premised upon an assumption that the cardiac rate is always faster than the respiratory rate. However, in certain (usually clinically catastrophic) situations, this assumption may not hold true.

For instance, there are clinical situations in which the lower frequency slower peak is actually reflecting the heart rate and the next-faster higher frequency peak is actually reflecting the respiratory rate. For example, with reference to FIG. 4C, there are certain situations where the peak (P₁) with the lowest frequency (F₁) corresponds to the heart rate of the patient 102, and the peak (P₀) with the next-lowest frequency (F₀) corresponds to the respiratory rate of the patient 102. For example, graphically, the respiratory peak (P₀) is to the right of the heart rate peak (P₁) on the x-axis in FIG. 4C. This situation may be commonly referred to as reversal of respiratory and cardiac peaks.

Furthermore, signal noise occurring coincidently near a typical heart rate could also make it difficult to distinguish a cardiac peak from a respiratory peak. Therefore, identifiers other than the expected respective x-axis locations of the F₀ and/or F₁ peaks based on typical heart rates and/or respiratory rates would be helpful.

In particular embodiments, the PIVA system 100 utilizes additional waveforms and other information, in addition to the relative x-axis location of the F₀ and F₁ peaks (which are typically the two lowest frequency peaks), to increase the certainty of which waveform corresponds to the pulse waveform, or at least more readily identify situations in which the F₀ and F₁ peaks may not reliably represent the respiratory and pulse rate waveforms.

In an embodiment, and with reference to FIG. 4C, a method for distinguishing respiratory peaks from cardiac peaks includes identifying a substantially-repeating first peak frequency (F₀) associated with local maxima of the frequency-domain peripheral venous pressure signal. The method further includes identifying a substantially-repeating second peak frequency (F₁) associated with local maxima of the frequency-domain peripheral venous pressure signal, distinct from the identified first peak frequency. One of the first peak frequency (F₀) and second peak frequency (F₁) is a respiratory signal and the other of the first peak frequency (F₀) and second peak frequency (F₁) is a cardiac signal. The method further includes identifying at least a substantially-repeating first harmonic (F₂). As will be determined, the substantially-repeating first harmonic (F₂) is associated with one of the first peak frequency (F₀) or the second peak frequency (F₁).

In an embodiment, when there is an indication of a change in x-axis location of either F₀ or F₁, the identification of the first or second peak frequency is confirmed by verifying the presence or absence of the associated first harmonic (F₂). For example, because the location, along the x-axis, of the first harmonic peak is expected to be double the x-axis location for the heart rate signal peak x-axis location, verifying the presence or absence of the associated first harmonic (F₂) includes scanning for the presence of the associated first harmonic (F₂) at a location along an x-axis of a PIVA signal pattern that is double the value of an x-axis location of the one of the first peak frequency or the second peak frequency. As used herein, double is intended to encompass exactly double as well as within ±15% of double.

In an alternate embodiment, the step of verifying the presence or absence of the associated first harmonic may instead be performed by a trigger different from an indication in change in x-axis location of either F₀ or F₁. For instance, the verification step may be performed upon each iteration of the FFT calculation employed to generate the plotted PIVA signal.

The indication of a change in x-axis location of either the first or second peak frequencies (or a routine verification step performed upon each FFT calculation) can include identification of an inversion in relative amplitude of the first and second peak frequencies.

The method may further including identifying a substantially-repeating second harmonic (F₃) associated with the same one of the first peak frequency or the second peak frequency with which the identified first harmonic F₂ is associated. Likewise, confirming the identification of the first or second peak frequency can further include verifying the presence or absence of the associated second harmonic (F₃). For example, because the location, along the x-axis, of the second harmonic peak is expected to be triple the x-axis location for the heart rate signal peak x-axis location, verifying the presence or absence of the associated second harmonic (F₃) includes scanning for the presence of the associated second harmonic (F₃) at a location along an x-axis of a PIVA signal pattern that is triple the value of an x-axis location of the one of the first peak frequency or the second peak frequency. As used herein, triple is intended to encompass exactly triple as well as within ±15% of triple.

In an embodiment, the mathematical relationship between x-axis locations of F₁, F₂, and F₃ are used to confirm that the F₁ peak represents the heart rate (even though graphically, the respiratory peak (P₀) is to the right of the heart rate peak (P₁) on the x-axis). For example, regardless of the respiratory peak (P₀) and the heart rate peak (P₁) relative positions on the x-axis, it is determined that the heart rate peak is ½ the distance from the associated first harmonic (F₂) and is ⅓ the distance from the associated second harmonic (F₃), according to the mathematical relationship identified above.

In a situation where the F₂ and F₃ peaks are either not found, or are not found at their expected locations, this would be indicative of the F₁ peak possibly not representing the heart rate, and a practitioner could be alerted to the fact that an F₀/F₁ reversal may have occurred.

In an embodiment, the method disclosed above may be performed by any of the PIVA system 100, the PIVA device 130, and the integrated fluid source 140.

Related Determination of Respiratory Metrics

In another embodiment, respiratory metrics may be determined, and tracked, from sensed pressure waveforms. For example, a respiratory frequency may be identified and evaluated to determine, among other things, risk of atelectasis, compliance with inspiratory spirometry, or respiratory insufficiency in the patient.

FIG. 6 illustrates a block diagram of an exemplary PIVA system 600 for use in measuring, analyzing, and responding to peripheral venous blood pressure of a patient 602. The exemplary PIVA system 600 is, in many ways, similar to the PIVA system 100 described above. It should be appreciated that PIVA system 600 can perform the same methods, processes, analysis, and calculations as that of PIVA system 100. For example, the PIVA system 600 may measure a pressure signal associated with the patient's peripheral vein, analyze the pressure using PIVA techniques to identify key frequency components of the pressure signal, and analyze the key frequency components of the pressure signal to determine patient status based upon one or more metrics, as discussed below.

The PIVA system 600 includes an IV tube 604 in fluid connection with the circulatory system of the patient 602. Specifically, a venous access device 606 may be inserted into a peripheral vein 608 of the patient 602 at an access point. The venous access device 606 may include a needle, catheter (such as a peripherally inserted central catheter), cannula, or other means of establishing a fluid connection between the IV tube 604 and the peripheral vein 608. The venous access device 606 may be a separate component connected to the IV tube 604 or may be formed as an integral portion of the of the IV tube 604. In either case, the venous access device 606 may include a terminal end inserted into the peripheral vein 608 at the access point and a connecting end that connects to a primary portion of the IV tube 604. The primary portion of the IV tube 604 may serve as a conduit between the venous access device 606 and a fluid source 610.

The fluid source 610 may include a fluid container and a flow controller. According to various embodiments, the flow controller may include a pump, such as an infusion pump, a dialysis pump, and/or any other medical fluid pump. The flow controller may also include any flow restrictor capable of preventing or otherwise controlling the rate that the fluid flows out of the fluid contained. For example, the flow restrictor may be a roller clamp, a valve, or any other device capable of being controlled to restrict the flow of the fluid. In some embodiments, the flow controller may be operated to adjust the flow restrictor to permit fluid within the fluid container to flow into the IV tube 604 and/or adjust the rate that the fluid within the fluid container flows into the IV tube 604.

At some point along the primary portion of the IV tube 604, a pressure sensor 612 may be disposed in fluid connection with the interior of the IV tube 604. The pressure sensor 612 is thus also in fluid connection with the peripheral vein 608 of the patient through the IV tube 604 and the venous access device 606. The pressure sensor 612 is thereby enabled to measure pressure changes in the peripheral venous system of the patient 602 based upon changes in the fluid pressure within the IV tube 604. In other embodiments, the pressure sensor 612 may not be disposed in fluid connection with the interior of the IV tube 604. For example, the pressure sensor 612 may instead be attached to the exterior of the IV tube 604 and thereby disconnected from the fluid source 610. As another example, the pressure sensor 612 may be coupled to the IV tube 604 via a flexible membrane that provides a sterile barrier between the fluids and the pressure sensor 612. The pressure sensor 612 may, in some such embodiments, measure pressure based upon acoustic or optical phenomenon at the sensor location. In some embodiments, the pressure sensor 612 may be disposed at a terminating end (i.e., a capped off end) of an IV tube 604 inserted specifically for the purpose of measuring pressure within the peripheral vein 608, in a manner similar to the device illustrated above in FIG. 2. In still other embodiments, other sensors may be used instead of the pressure sensor 612, such as force sensors, sound sensors, and sensors that need not be in fluid contact with the peripheral vein 608 of the patient through the IV tube 604 and the venous access device 606. In yet further embodiments, the sensor 612 may be disposed within a portion of a needle, catheter, or other venous access device 606 that is inserted within the peripheral vein 608 of the patient 606, such that PVP may be measured in situ within the peripheral vein 608. Such in situ measurement is advantageous inasmuch as it obviates the effect of variation in temperature, fluid viscosity/density, access device resistance, and other factors on the unmodified transmission of pressure waveforms within the IV tube 604.

In various embodiments, the pressure sensor 612 may be positioned at various distances from the access point of the peripheral vein 608, from a location within the peripheral vein 608 or a location proximate to the connecting end of the venous access device 606 to a position proximate to the fluid source 610 or at a terminating end of the IV tube 604. The pressure sensor 612 is illustrated as being at an intermediate location along the length of the IV tube 604 in order to illustrate better the various components of the PIVA system 600. In some embodiments, the pressure sensor 612 may directly measure fluid pressure within the IV tube 604. Specifically, the pressure sensor 612 may include a transducer that provides an electronic pressure signal indicative of the pressure detected by the transducer to an analysis component 614 via a connection 622. The electronic pressure signal may be an analog electrical signal directly provided by the transducer or may be a preprocessed digital signal indicating pressure values based upon the transducer interface with the primary portion of the IV tube 604. In embodiments in which the pressure sensor 612 is not in fluid connection with the IV tube 604 or the peripheral vein 608, the pressure sensor 612 may nonetheless include one or more transducers to generate electronic signals associated with the PVP. For example, the pressure sensor 612 may use one or more microphones disposed to detect sound at an exterior surface of an IV tube 604 to generate electronic pressure signals indicative of pressure within the IV tube 604 as a proxy for PVP within the peripheral vein 608.

The analysis component 614 is communicatively connected to the pressure sensor 612 to receive the electronic pressure signal via the connection 622. The analysis component 614 may include general-purpose or special-purpose processing hardware, such as microprocessors or special-purpose analysis circuits. As shown, the analysis component 614 may include one or more units for performing the PIVA analysis. A response unit 616 may identify and control responses based upon the pressure data from the pressure sensor 612. The response unit 616 may control the presentation of alarms, the operation of the fluid source 610, such as by controlling the rate of fluid flow, or the operation of a ventilator 652, such as by controlling the rate of respiration. To determine appropriate responses, the response unit 616 may receive evaluation data from an evaluation unit 618, which may include metrics determined from the electronic pressure signal. The evaluation unit 618 may obtain pressure values (or signal values directly or indirectly associated with PVP) from the electronic pressure signal and evaluate the pressure values to determine information regarding the patient 602, such as blood volume metrics, respiratory metrics, or other metrics as described in further detail below. The information generated by the evaluation unit 618 may also be stored or presented for patient monitoring. In alternative embodiments, additional, fewer, or alternative units may be included. For example, the evaluation unit 618 may perform the functions ascribed to the response unit 616 herein. As another example, the analysis conducted by the analysis component 614 may be performed at any point between the signal source, such as the pressure sensor 612, and a display unit, such as a monitor 620.

The analysis component 614 may be communicatively connected to the monitor 620 via a connection 626 in some embodiments. The monitor 620 may be a separate monitor for displaying information regarding the patient or may be incorporated into another device, such as a dialysis pump, a patient-controlled analgesia pump, or an infusion or other pump associated with IV tube 604, or other flow controller of the fluid source 610. The monitor 620 may also be communicatively connected to the fluid source 610 via a connection 628 to receive and display information associated with the fluid source 610. In some embodiments, the monitor 620 may be used to control the operation of the fluid source 610, such as by adjusting the flow controller to alter fluid flow rate, duration of operation, mode of operation, etc. In some embodiments, the monitor 620 may also be communicatively connected to a ventilator control 650 via a connection 629 to receive and display information associated with the respirator 652. The ventilator control 650 may control the operation of the ventilator 652 via a connection 651. For example, the ventilator control 650 may control a rate of respiration, a volume of oxygen per respiration, a respiratory pressure, etc. In some embodiments, the monitor 620 may be used to control the operation of additional or alternative respiratory equipment, including an extracorporeal blood oxygenation unit (such as a continuous renal replacement therapy unit). The analysis component 614 may similarly be communicatively connected to the fluid source 610 via connection 624 in some embodiments. The analysis component 614 may receive information regarding operation of the fluid source 610 for use in evaluating the patient by the evaluation unit 618. The response unit 616 may also communicate with the fluid source 610 to control operation of the fluid source 610 and/or the ventilator control 650 to control operation of the ventilator 652 in response to information regarding the patient determined based upon the electronic pressure signal from the pressure sensor 612.

The various connections 622, 624, 626, 628, 629, and 651 may each be wired or wireless connections in various embodiments. Moreover, some or all of the connections 622, 624, 626, 628, 629, and 651 may be internal to devices, such as a PIVA device 630 or a PIVA-integrated fluid control device 640. The PIVA device 630 may incorporate the pressure sensor 612 and analysis component 614 (along with associated connections) into a device that may be attached to or within the IV tube 604 to perform PIVA monitoring of the patient 602. The PIVA-integrated fluid control device 640 may include a computer-controlled fluid reservoir (e.g., the fluid container of the fluid source 610) and/or pump (e.g., the flow controller of the fluid source 610) configured to utilize PIVA monitoring of the patient 602 in controlling the flow of fluids from the fluid container of the fluid source 610. Like the PIVA device 630, the PIVA-integrated fluid control device 640 may include the pressure sensor 612 and analysis component 614, along with the fluid source 610 and the monitor 620 (along with associated connections). Alternative embodiments may include additional, fewer, or alternative components in alternative configurations.

With reference to the PIVA system 600 described herein, and the methods disclosed above, respiratory status metrics may be readily determined. The respiratory status metrics may include metrics indicative of a respiratory volume/depth and/or a respiratory effort. As described above, respiratory depth may correlate to the magnitude of the peak (P₀) associated with the respiratory frequency (F₀). Accordingly, respiratory volume may correlate with the product of the magnitude of the peak (P₀), associated with the respiratory frequency (F₀), and the respiratory frequency (F₀). These values may be calculated via PIVA system 600.

The respiratory depth and/or volume metrics may be indicative of a risk of atelectasis, patient compliance with inspiratory spirometry, respiratory insufficiency, hyperventilation, and/or other respiratory conditions. Additionally, the magnitude of the peak (P₀), associated with the respiratory frequency (F₀), may be associated with a respiratory effort metric.

In addition to the magnitudes of the peaks (P_(N)), the evaluation unit 618 may also analyze the shape of the frequency-domain representation of the PVP signal at one or more of the frequencies (F_(N)) to determine the one or more aspects of patient status. For example, the width of the lowest frequency peak (P₀) may be determinative of respiratory consistency. Generally speaking, consistent breathing patterns occur at a regular frequency. Accordingly, when the patient 602 exhibits consistent breathing patterns, the lowest frequency peak (P₀) contains fewer frequency variations. As such, when the lowest frequency peak (P₀) is narrow (e.g., a narrow parabolic peak), the evaluation unit 618 may determine that the patient 602 has a consistent breathing pattern. Conversely, when the patient 602 exhibits inconsistent breathing patterns, there is generally higher variation the respiration frequency. Accordingly, the lowest frequency peak (P₀) for this patient will tend to be wider, incorporating the variance in the respiration rate. Thus, when the lowest frequency peak (P₀) is wide (e.g., a wide parabolic peak), the evaluation unit 618 may determine that the patient 602 has an inconsistent breathing pattern. In order to calculate the width of the peak, the evaluation unit 618 may use any generally known peak-width calculation techniques, such as, for example, a full-width half maximum (FWHM) width calculation.

In another aspect, the evaluation unit 618 may also analyze the slope of the either the rising-edge or falling-edge of the peaks (P_(N)) to determine the one or more aspects of the patient status. For example, similar to the width of the lowest frequency peak (P₀), the slopes of the lowest frequency peak (P₀) may also indicate respiratory consistency. Generally, shallower slopes correspond to wider peaks. However, comparing the rising-edge slope and the falling-edge slope of the lowest frequency peak (P₀) may indicate additional aspects of the patient status. To this end, when the respiratory rate of the patient 602 speeds up, the lowest frequency peak (P₀) may begin to widen on the falling-edge to incorporate the frequency content associated with the faster respiratory rate. Thus, this quickening trend in breathing pattern may be detected by determining that the lowest frequency peak (P₀) has a shallower falling-edge slope as compared to the rising-edge slope. Conversely, as the respiration rate of the patient 602 slows down, the lowest frequency peak (P₀) may shift such that the peak encompasses the lower respiration frequency content. Accordingly, this slowing trend in breathing pattern may be detected by determining that the lowest frequency peak (P₀) has a steeper falling-edge slope as compared to the rising-edge slope.

In still other aspects, a respiratory effort metric, such as work of breathing, may correlate to a ratio between the magnitude of the peak (P₀) corresponding to the respiratory frequency (F₀) to the magnitude of the peak (P₁) corresponding to the heart rate frequency (F₁). Additionally, a respiratory effort metric associated with the presence of a respiratory obstruction may be determined by analyzing the shape of the peak (P₀) corresponding to the respiratory frequency (F₀). The obstruction may either be an inspiratory obstruction, such as croup, or it may be an expiratory obstruction, such as asthma or chronic obstructive pulmonary disease.

It should be appreciated that while the pressure signal analysis techniques described herein have been generally described with respect to PIVA system 600, these pressure signal analysis techniques may be applied to pressure signals detected by other systems. To this end, in some embodiments, the pressure sensor 612 may be located in alternative channel of the circulatory system as compared to a peripheral intravenous channel. Such alternative channels include a central venous channel, an arterial channel, as well as a peritoneal dialysis channel. As described above, some of these alternative channels are capable of detecting lower-than-respiratory frequencies, such as pressure fluctuations associated with gut behavior. Accordingly, as described elsewhere herein, in these embodiments, the pressure sensor and/or a device communicatively coupled to the pressure sensor may transmit to the evaluation unit 618 a peak index corresponding to the respiratory frequency.

More generally, any of the systems disclosed herein, such as PIVA system 100 and PIVA system 600 can perform any of the methods disclosed herein.

As used in this specification, including the claims, the term “and/or” is a conjunction that is either inclusive or exclusive. Accordingly, the term “and/or” either signifies the presence of two or more things in a group or signifies that one selection may be made from a group of alternatives.

The many features and advantages of the present disclosure are apparent from the written description, and thus, the appended claims are intended to cover all such features and advantages of the disclosure. Further, since numerous modifications and changes will readily occur to those skilled in the art, the present disclosure is not limited to the exact construction and operation as illustrated and described. Therefore, the described embodiments should be taken as illustrative and not restrictive, and the disclosure should not be limited to the details given herein but should be defined by the following claims and their full scope of equivalents, whether foreseeable or unforeseeable now or in the future. 

1. A blood volume evaluation system for monitoring a patient using a measurement associated with a peripheral venous pressure (PVP) within a peripheral vein of a circulatory system of the patient, comprising: a PVP sensor including a transducer disposed adjacent to or connected to an intravenous (IV) tube in fluid connection with the peripheral vein and configured to generate an electronic signal associated with the PVP; and an evaluation unit, including a computer processor communicatively connected to the PVP sensor to receive the electronic signal and a memory storing non-transitory computer-readable instructions that, when executed by the computer processor, cause the evaluation unit to: obtain a time-domain PVP signal, including a plurality of values of the electronic signal over a sample period; apply a transformation to the time-domain PVP signal to generate a frequency-domain PVP signal; identify a plurality of frequencies associated with local maxima of the frequency-domain PVP signal, including a heart rate frequency (F₁), a first harmonic frequency (F₂) at a first harmonic of the heart rate frequency, and a second harmonic frequency (F₃) at a second harmonic of the heart rate frequency; and calculate a blood volume metric based at least in part upon the heart rate frequency (F₁), the first harmonic frequency (F₂), and the second harmonic frequency (F₃).
 2. The blood volume evaluation system of claim 1, wherein calculating the blood volume metric based at least in part upon the heart rate frequency (F₁), the first harmonic frequency (F₂), and the second harmonic frequency (F₃) includes the equation ${{PIVA}\mspace{14mu} {Score}} = {{c_{3}{\tanh \left( \frac{g_{0} + {g_{1}{mag}^{f\; 1}} + {g_{2}{mag}^{f\; 2}} + {g_{3}{mag}^{f\; 3}}}{2} \right)}} + {c_{2}{\tanh \left( \frac{h_{0} + {h_{1}{mag}^{f\; 1}} + {h_{2}{mag}^{f\; 2}} + {h_{3}{mag}^{f\; 3}}}{2} \right)}} + {c_{1}{\tanh \left( \frac{i_{0} + {i_{1}{mag}^{f\; 1}} + {i_{2}{mag}^{f\; 2}} + {i_{3}{mag}^{f\; 3}}}{2} \right)}} + c_{0}}$ wherein each of c₀, c₁, c₂, c₃, g₀, g₁, g₂, g₃, h₀, h₁, h₂, h₃, i₀, i₁, i₂, and i₃ are constants, wherein mag^(f1) is the magnitude of the heart rate frequency (F₁), wherein mag^(f2) is the magnitude of the first harmonic frequency (F₂), and wherein mag^(f3) is the magnitude of the second harmonic frequency (F₃).
 3. The blood volume evaluation system of claim 1, wherein the blood volume metric is an indication of one of the following: hypovolemia, hypervolemia, or euvolemia.
 4. The blood volume evaluation system of claim 1, further comprising an alarm unit configured to present alarms to a user, and wherein the instructions further cause the evaluation unit to communicate an alarm indicator to a presentation unit when the blood volume metric indicates the blood volume of the patient is abnormal, the alarm indicator causing the presentation unit to present an alarm as at least one of (i) an electronic message or a visual notification via a display or (ii) an audible notification via a speaker.
 5. The blood volume evaluation system of claim 4, wherein the alarm includes a recommendation of an action to be taken in response to the calculated blood volume metric.
 6. The blood volume evaluation system of claim 1, wherein the electronic signal is an electronic pressure signal indicative of pressure within the IV tube.
 7. The blood volume evaluation system of claim 6, wherein: the transducer of the PVP sensor is a pressure sensor and is disposed in fluid connection with the interior of the IV tube; and the interior of the IV tube is in fluid connection with the circulatory system of the patient via the peripheral vein.
 8. A device for measuring and evaluating blood volume of a patient, comprising: a peripheral venous pressure (PVP) sensor, including a transducer configured to generate an electronic signal associated with a pressure within a peripheral vein of a circulatory system of the patient; and an evaluation unit, including a computer processor communicatively connected to the PVP sensor to receive the electronic signal and a memory storing non-transitory computer-readable instructions that, when executed by the computer processor, cause the evaluation unit to: obtain a time-domain PVP signal, including a plurality of values of the electronic signal over a sample period; apply a transformation to the time-domain PVP signal to generate a frequency-domain PVP signal; identify a plurality of frequencies associated with local maxima of the frequency-domain PVP signal, including a heart rate frequency (F₁), a first harmonic frequency (F₂) at a first harmonic of the heart rate frequency, and a second harmonic frequency (F₃) at a second harmonic of the heart rate frequency; and calculate a blood volume metric based at least in part upon the heart rate frequency (F₁), the first harmonic frequency (F₂), and the second harmonic frequency (F₃).
 9. The device of claim 8, wherein the blood volume metric is an indication of one of the following: hypovolemia, hypervolemia, or euvolemia.
 10. The device of claim 8, wherein obtaining the time-domain PVP signal includes receiving and storing the plurality of values of the electronic signal over the sample period.
 11. The device of claim 8, wherein the sample period is a fixed duration that is continuously updated on a rolling basis as new values of the electronic signal are obtained.
 12. The device of claim 8, wherein the transformation is a fast Fourier transformation.
 13. The device of claim 8, wherein the blood volume metric is further determined at least in part based upon at least one or more additional harmonic frequency associated with additional harmonics of the heart rate frequency (F₁), including at least a third harmonic frequency (F₄).
 14. The device of claim 8, wherein the blood volume metric is determined based at least in part upon an increase or decrease in a magnitude of the frequency-domain PVP signal associated with the first harmonic frequency (F₂).
 15. A method of measuring and evaluating blood volume of a patient using a measurement associated with a peripheral venous pressure (PVP) within a peripheral vein of a circulatory system of the patient, comprising: monitoring, by a transducer of a PVP sensor, a physical phenomenon associated with the PVP within the peripheral vein of the circulatory system of the patient; generating, by the PVP sensor, an electronic signal associated with the PVP over a sample period, the values of the electronic signal over the sample period forming a time-domain PVP signal; applying, by a processor of an evaluation unit, a transformation to the time-domain PVP signal to generate a frequency-domain PVP signal; identifying, by the processor of the evaluation unit, a plurality of frequencies associated with local maxima of the frequency-domain PVP signal, including a heart rate frequency (F₁), a first harmonic frequency (F₂) of the heart rate frequency, and a second harmonic frequency (F₃) of the heart rate frequency; and calculating, by the processor of the evaluation unit, a blood volume metric based at least in part upon the heart rate frequency (F₁), the first harmonic frequency (F₂), and the second harmonic frequency (F₃).
 16. The method of claim 15, wherein the blood volume metric is an indication of one of the following: hypovolemia, hypervolemia, or euvolemia.
 17. The method of claim 15, wherein the plurality of frequencies further include one or more additional harmonic frequencies associated with additional harmonics of the heart rate frequency (F₁), including at least a third harmonic frequency (F₄).
 18. The method of claim 17, wherein the one or more additional harmonic frequencies are identified as the frequencies associated with local maxima of the frequency-domain PVP signal nearest to integer multiples of the heart rate frequency (F₁).
 19. The method of claim 18, the blood volume metric is further determined at least in part based upon at least one of the additional harmonic frequencies, including at least the third harmonic frequency (F₄).
 20. The method of claim 19, wherein the blood volume metric is determined based at least in part upon a harmonics ratio between the first harmonic frequency (F₂) and at least one of the second harmonic frequency (F₃) and the third harmonic frequency (F₄). 